This achievement requires you to defeat FRANK-Y, one of the two bosses in the Factory area. Similarly to the B-DSS Knight, you will see a special symbol on the map which signifies his boss room. This room is located in the 'run down' area of the factory. As you progress, rooms will start having grey walls and other aesthetics that resemble a derelict area. Make sure to prepare Heart before the fight as you cannot leave the boss room once you enter. FRANK-Y is vulnerable to fire weapons and so the fight will be much easier if you have one.
FRANK-Y can be hit on the small robot on his front, or his 'legs'. Dealing enough damage to one of his legs will cause the armour fall away from it and the boss will be stunned for a short while for you to get some free hits in on him.
For his attacks, he has an overhead slash that he repeats three times and each slash will track your movement. To avoid it you can simply roll away as he is swinging his arm down and then punish him. Also, he can move his whole body to the side which indicates that he will do a grounded slice which is easily evaded just by jumping at the right time. FRANK-Y can also perform a slam on the ground which produces a shockwave, in order to dodge this, you will have to jump at the right time. Another of his attacks is a short rush forward which is telegraphed by him leaning back; it can be avoided by double jumping over him or moving out of range.
FRANK-Y's attacks are well telegraphed and easily avoided, but this does not mean you should be relaxed during the fight. Getting hit once will cause you take at least two hearts of damage and so you should hit him when you can and get out as soon as you see the yellow exclamation points above his head. After defeating him there will be a hole in the floor for you to jump down. This will give you the achievement as well as spawning you in the Sewer area of the game.